Bevel modifier buckling bug
Bad news
This is a bug in Blender 2.79b. It has already been reported.
Good news
There has already been a fix for this. If you download Master at https://builder.blender.org/download/ and check with the exact same file, the Bevel should work as expected. The same fix has also been applied to the 2.80 series of Blender as well.